refactor(EventHandler): add missing return type to onInstall method

src/QUI/Test/EventHandler.php

@@ -31,7 +31,7 @@ public static function onUserGetAvatar(UserInterfaces $User)
     /**
      * @param Package $Package
      */
-    public static function onInstall(Package $Package)
+    public static function onInstall(Package $Package): void
     {
         Log::writeRecursive('ON INSTALL');
         Log::writeRecursive($Package->getName());